This invention relates to a fault locating system which is used for locating a fault in a communication network which has a hierarchy and which is therefore divisible into a plurality of layers graded from an uppermost layer to a lowermost one.
Heretofore, a fault detection system is disclosed in Japanese Unexamined Patent Publication No. Syo 60-250745, namely, 250745/1985 and is applicable to a hierarchical communication network. With this previous structure, a wide variety of communication units are used in the hierarchical communication network and divided into a plurality of layers from an uppermost layer to a lowermost one. In such a communication network, each signal is multiplexed into a multiplexed signal in each communication unit except for the lowermost layer.
Now, when faults occur in communication units which may be called faulty communication units, alarm signals are produced from the faulty communication units. Such alarm signals are collected and coded into an alarm pattern which is composed of a plurality of bits and which may be represented by AP. In this event, each bit takes a logic "1" level and a logic "0" level when a fault occurs or does not, respectively.
Herein, it is assumed that zeroth through I-th communication units are placed at zeroth through I-th locations 0 to I in the communication system and are represented by a.sub.0 to a.sub.I, respectively. In this connection, an i-th communication unit is depicted at a.sub.i. When a fault occurs in the i-th communication unit, the alarm signal is assumed to be represented by b.sub.i. Under the circumstances, it is to be noted that the alarm pattern AP can be expressed by: EQU AP=b.sub.I 2.sup.I +b.sub.I-1 2.sup.I-1 + . . . +b.sub.i 2.sup.i + . . . +b.sub.1 2.sup.1 +b.sub.0,
where b.sub.i equals 0 and 1 in a normal state and in a faulty state, respectively.
Let a fault occur in a certain one of the communication units that is present in a certain layer in the communication network wherein a signal is multiplexed into the multiplexed signal determined by the hierarchy of the communication units. In this event, an alarm pattern is formed due to such a fault, as mentioned above. Moreover, an influence of such a fault is spread or repercussed from the certain layer to all the communication units connected to lower layers. This shows that the alarm signal is spread or repercussed over the lower layers to cause spread patterns to occur in the communication units connected to the lower layers. Hereinafter, such alarm and spread patterns which actually appear on occurrence of a fault will be collectively called actual alarm patterns.
In order to locate the fault in the conventional communication network spread patterns are previously memorized which will appear at each of points in the communication network when faults occur at the respective points. Such spread patterns may be referred to as forecast spread patterns and are successively compared with the actual alarm patterns to locate the fault or to specify each point of the fault.
With this structure, it is impossible to locate a fault when an actual alarm pattern is not coincident with any one of the forecast spread patterns. In addition, the forecast spread patterns vastly increase as the scale of the communication network becomes large. This necessitates an increase of a memory area for memorizing the forecast spread patterns. Moreover, a very long time is required to successively compare the actual alarm pattern with the forecast spread patterns and to locate the fault.